This invention relates to a variable optical attenuator for controllably attenuating optical signals. In particular, this invention relates to a variable optical attenuator in the form of a graded holes filter that is substantially wavelength independent and substantially polarization independent.
There are numerous optical attenuators known in the art. Ideally, an optical attenuator provides means for controlling the intensity of a beam of light without appreciably affecting its other properties.
Traditionally, variable apertures or irises have been used to regulate the cross-section of a beam of light, and hence its total energy content. However, most single aperture attenuators that block a portion of the optical signal are wavelength dependent and polarization dependent, thus limiting the usefulness of these devices in many applications.
Optical attenuators of the wire-grid type are also known and are described, for example, in U.S. Pat. No. 3,620,599 to Arthur Ashkin, hereby incorporated by reference. These type of optical attenuators operate under the principles of diffraction. The wire-grid comprises a plurality of uniformly spaced parallel reflective wires, wherein the wire diameter and spacings therebetween are selected to be one to three orders of magnitude greater than the wavelength of the optical radiation to be attenuated. Variable attenuation is accomplished via rotation of the grid about an axis parallel to said wires. Although, these optical attenuators perform well under high power conditions, numerous problems arise from scattering and/or backreflection, which degrades the quality of the transmission and leads to feedback noise. When these attenuators are close to a laser, the reflections also contribute to reduced laser performance.
More commonly, variable attenuators are continuously variable neutral density filters, wherein optical density is a varying function of position over the area of the filter. These are generally formed from an optical glass substrate with a metallic coating that has a density that increases as it progresses across the surface of the filter. Attenuation is effected by absorption and reflection of the e optical signal as the filter i s moved relative to the optical beam. Generally, the-movement of such a filter is rotation around the axis of the attenuated beam.
A continuously variable fibre optic attenuator wherein the density gradient varies along its length is disclosed in U.S. Pat. No. 4,904,044 to Tamulevich, hereby incorporated by reference. The attenuator uses a flexible filter of varying optical density which is oriented in an optical coupling region between two optical fibres. The filter is displaced in a manner to vary the filter density in the optical coupling region and thereby vary the attenuation across the device. A resistor coupled to the attenuator provides means for calibration of the attenuator to provide a highly accurate and reproducible attenuation.
Although continuously variable neutral density filters perform satisfactorily, they are expensive to manufacture and exhibit some weakness in spectral absorption, reflection, and refraction characteristics.
In particular, since neutral density features exhibit high absorption characteristics the t temperature of the metal coating reaches high levels, thus inducing some wear over time (e.g., delamination, oxidation, annealing, etc.). As a result, the intrinsic characteristics of the filter are affected, e.g. density. Furthermore, disturbances due to interference of the reflected radiation resulting from irregularities on the surface of the filter, are a further limitation, particularly when the temperature of the filter increases, as found in high light intensity applications.
Moreover, the application of the metallic coating requires a high degree of accuracy, and frequently varies from batch to batch. Furthermore, it is extremely difficult to manufacture a neutral density filter wherein the metallic layer is deposited in a manner that produces a substantially linear relationship between attenuation and position of the filter.
It is an object of this invention to provide an optical attenuator that obviates most or all of the above mentioned disadvantages, and that is less complex and less costly to manufacture.
It is yet a further object of the invention to provide an optical attenuator that is substantially wavelength independent, substantially polarization independent, and is useful for high power applications.
The instant invention provides a variable optical attenuator that is substantially wavelength independent and substantially polarization independent. Similar to optical attenuators of the wire-grid type, the graded holes filter of the instant invention operates under the principles of diffraction. Specifically, zeroth order diffracted light transmitted through the filter provides the wavelength independent attenuated radiation.
In accordance with the invention, there is provided, a variable optical attenuator for attenuating a beam of light comprising an optical signal having a diameter less than about 1 mm, comprising:
a filter comprising a sheet having a plurality of apertures discretely arranged for providing a transmission gradient along a path defining a gradient axis of the sheet, the plurality of apertures having a predetermined shape, size and distribution, and having a length and a distance therebetween that is substantially smaller than the diameter of the beam of light;
an input port for launching the beam of light towards the filter at a predetermined angle;
an output port for receiving substantially focussed light from the filter; and,
means for providing relative movement between the beam of light and the filter for effecting variable attenuation of the beam of light in dependence upon a position of said filter relative to said beam of light.
In accordance with the invention there is further provided, a variable optical attenuator for attenuating a beam of light comprising an optical signal having a diameter less than about 1 mm, comprising:
a filter comprising a sheet having a plurality of substantially opaque regions and a plurality of substantially transparent regions discretely arranged thereon such that a percentage of transparent region per unit area increases along a dimension of the filter, the unit area having a diameter equivalent to the diameter of the beam of light, the plurality of substantially transparent regions having a predetermined shape, size and distribution, and at least a portion of the substantially transparent regions having a length and a distance therebetween substantially smaller than the diameter of the beam of light;
an input port for launching the beam of light towards the filter at a predetermined angle;
an output port for receiving substantially focussed light from the filter; and,
means for providing relative movement between the beam of light and the filter for effecting variable attenuation of the beam of light in dependence upon a position of said filter relative to said beam of light.
In accordance with the invention there is provided, a method of variably attenuating a beam of light comprising an optical signal having a diameter less than about 1 mm comprising the steps of:
(a) irradiating a filter comprising a sheet having a plurality of substantially opaque regions and a plurality of substantially transparent regions discretely arranged thereon such that a percentage of transparent region per unit area increases along a dimension of the filter, the unit area having a diameter equivalent to the diameter of the beam of light, the plurality of substantially transparent regions having a predetermined shape, size and distribution, and at least a portion of the substantially transparent regions having a length and a distance therebetween substantially smaller than the diameter of the beam of light;
(b) relatively moving the filter and the beam of light for effecting variable attenuation of the beam of light in dependence upon a required degree of attenuation.